Phantoms, composed of tissue substitutes, are used as a proxy for real subjects by scientists, health physicists, and physicians to study how ionizing radiation material, especially gamma-emitting radionuclides, is deposited within a subject (animal or human) exposed to ionizing radiation material. Primary functions of phantoms include dosimetry, calibration, and imaging. Within each functional category, primary types of phantoms include anthropomorphic (body), standard, and reference. It will be apparent to those skilled in the art of phantoms that the present invention is useful for any category, any type or combination thereof.
The fundamental shortcoming of any proxy is the uncertainty introduced by differences between the proxy and the real subject. For phantoms, it is desired to match (a) geometry, (b) radiation response properties of tissue, and (c) the interface between phantom and detector. Of these three criteria, the second, radiation response properties, is the most difficult to match and is the source of greatest uncertainty. Within the radiation response properties criteria, the main parameter scientists seek to match between phantoms and real subjects is the attenuation coefficients which are a function of the chemical composition, the effective atomic weight, and the density of the phantom material.
Proxys are used both for whole body modeling and for specific organ modeling; for example, the lung. Because the various organs have different shapes, sizes, and biological/chemical composition, individual organs respond differently to ionizing radiation material. Hence, their proxys are necessarily different. This invention is specifically directed toward a proxy or phantom for modeling physical response to ionizing radiation material deposited in a lung.
Lung phantoms have been developed and relied upon for many years. Generally readily available plastics, for example polyurethane, have been used as well as other low density materials. However, for more accurate modeling, specific formulations have been developed including Temex having a polymerized rubber, polyvinyl chloride (PVC) "polyfoam", and Rando having a rigid filled epoxy. Again, however, for measurements of ionizing radiation having energies as low as 15 keV exposing tissues of thickness greater than 4 cm, yet different phantom formulations were needed.
At the Symposium on Advances in Radiation Protection Monitoring, Stockholm, Sweden, Jun. 26-30, 1978, authors RV Griffith et al., presented FABRICATION OF A TISSUE-EQUIVALENT TORSO PHANTOM FOR INTERCALIBRATION OF IN-VIVO TRANSURANIC-NUCLIDE COUNTING FACILITIES, described a phantom constructed of a
rib cage, chest plates, and various removable organs including lungs, heart, liver, kidneys, spleen, and tracheo-bronchial lymph nodes. Polyurethane with different concentrations of calcium carbonate was used to simulate the linear photon attenuation properties of human tissues of lean muscle, adipose muscle mixtures, and cartilage.
These phantom elements are prepared for calibration measurements by incorporating highly pure .sup.238 Pu, .sup.239 Pu, and .sup.241 Am, for simulating uniform deposition of ionizing radiation. In particular, the lung phantom material consisted of a mixture of (a) a two-component polyurethane, 1940D (black), obtained from CPR Division of Upjohn Corporation, but no longer available. The 1940D was a castable isocyanate polyurethane requiring addition of water for foaming. The Griffith et al. lung phantom was specifically 30% by weight of component A of the polyurethane and 68.4% by weight of component B, (b) 0.15% by weight water to generate foaming, 0.15% by weight acetone with transuranic tracer, 6.2% calcium carbonate added to component A to obtain proper x-ray transmission, and lanthanum nitrate as a carrier for the transuranic nuclides tracer. However, results were not duplicable without a second catalyst, stannous octoate, used to give a workable reaction rate, and a heated mold for providing uniform density (unpublished letter Jun. 1980 from R. Griffith to H. E. Palmer).
The Griffith et al. phantom material is made by first mixing the calcium carbonate and component A of the polyurethane. Component B, water and stannous octoate, are added and the foaming reaction begins. The acetone and lanthanum nitrate containing the transuranic tracer are mixed and added to the polyurethane, making a total mixture. The total mixture poured is into a mold. Final density is controlled by the amount of total mixture poured into the mold, and uniformity of density is controlled by preheating the mold to a predetermined temperature.
The calcium carbonate powder has a tendency to settle in the liquid foam. In order to suspend the calcium carbonate in the liquid foam, a thixotropic concentrate is prepared prior to the foaming process. The thixotropic concentrate is prepared by first mixing component B and calcium carbonate in a relative proportion of about 2:1, then passing the mixture through a three roll paint mill. Any excess moisture remaining from the milling is removed by heating the milled mixture to 120.degree. F. under vacuum.
It is necessary to mix and pour within about 30 sec because the foaming reaction begins as soon as parts A and B of the polyurethane are mixed, and the stannous octoate catalyst increases the reaction rate so that if the mixture is not poured within about 30 sec of mixing, then the amount of foaming renders the mixture un-pourable.
While this lung-phantom closely matched two actual lung parameters, 3869 cm.sup.3 phantom compared to 3915 cm.sup.3 reference man, and density, 0.28 g/cm.sup.3 phantom compared to 0.31 g/cm.sup.3 lung tissue (Griffith et al., Table III), it is neither shown how this lung phantom material compares to chemical composition of a real lung, nor shown what attenuation coefficients result from a range of radiation energies.
Further, as reported in the ICRL82655 Preprint, Polyurethane as a Base For a Family of Tissue Equivalent Materials, R. Griffith, 5th International Congress of IRPA, Jerusalem, Israel, 14 Mar. 1980, the 17.2 keV linear attenuation coefficient of the Griffith et al. lung phantom is 0.272 cm.sup.-1, while the ICRU 46 lung linear attenuation coefficient at 17.2 keV is 0.312 cm.sup.-1.
The ICRU 46 lung is specified as having a chemical composition of 10.5 weight percent carbon, 10.3 weight percent hydrogen, 3.1 weight percent nitrogen, 74.9 weight percent oxygen, plus trace amounts of phosphorus, sodium, sulphur, chlorine, and potassium, totalling to 1.2 weight percent.
It would be advantageous to identify a lung phantom that can be made from materials presently available, and that more closely matches geometry, density, linear attenuation and chemical composition of actual lung material.